AU2011347466A1 - Process for producing synthesis gas - Google Patents

Abstract

A process for controlling the carbon conversion of a gasifier fuelled with a carbonaceous feedstock by mixing in biomass, the process comprising the steps of (a) pressurizing the biomass and carbonaceous feedstock; (b) introducing the biomass and carbonaceous feedstock into the gasification reactor vessel; (c) partially oxidizing the carbonaceous feedstock/biomass with a molecular oxygen-comprising gas to obtain a synthesis gas comprising carbon monoxide and hydrogen; (d) measuring the C02 content of the syngas and comparing with a pre-determined value range; (e) adjusting the biomass/carbonaceous feedstock ratio by changing the biomass feed rate; wherein said biomass and carbonaceous feedstock comprises from 10 wt% to 50 wt% of biomass and wherein the level of biomass is adjusted within this range to control the carbon conversion.

Description

WO 2012/084953 - 1 - PCT/EP2011/073417 PROCESS FOR PRODUCING SYNTHESIS GAS The present invention relates to a process for producing synthesis gas stream from a carbonaceous feedstock, which process includes biomass fuel as a means to control the carbon conversion at a given gasification 5 temperature in a gasification process. WO 03/012013 describes a process for the incomplete combustion of domestic waste to produce syngas. The process as described may also include controlling the temperature in various parts of the reaction space by 10 controlling the temperature of the oxygen-containing gas fed thereto, controlling the rate of feed of the waste and the resulting ratio of the waste feed to the oxygen containing gas feed, and thermally insulating the reation space. 15 JP2002194363 describes a method for the pressurized entrained bed gasification of coal, by which biomass can be used to efficiently lower high heat generated by gasification of the coal, thereby reducing the occurrence of problems with the accumulation and fusion of ash on 20 the operation of a pressurized entrained bed gasification furnace. The method is characterized by introducing coal particles into the lower portion of the pressurized entrained bed gasification furnace to produce the high temperature coal gas, introducing a biomass fuel into the 25 upper portion to bring into contact with the high temperature coal gas and gasifying the biomass fuel. Thereby, the height of an exhaust heat recovery boiler of the gasification process can be controlled. In the present invention a carbonaceous feedstock 30 such as coal may be mixed with biomass prior to the mixture being introduced into the burner section of the WO 2012/084953 - 2 - PCT/EP2011/073417 gasifier. The mixing of biomass with the coal in such a manner helps to ensure synergy between the oxygen rich biomass and oxygen lean carbonaceous feedstocks. Due to the high oxygen content in the biomass feedstock, the 5 requirement of a moderator gas to obtain full conversion of the carbonaceous feedstock at a given gasification temperature is reduced and can even be eliminated at sufficient high fractions of biomass in the coal/biomass mixture. As moderator gas steam or carbon dioxide or a 10 combination thereof is typically used. An object of the present invention is to provide a simplified, energy efficient and renewable means of controlling the carbonaceous feedstock conversion by mixing in a biomass fuel. Biomass in general has a high 15 oxygen content and can provide part of the oxygen required for full conversion of the carbonaceous feedstock, which in turn reduces the requirement of a moderator gas, such as steam. A certain minimum biomass fraction in the biomass/carbonaceous feedstock mixture is 20 required to eliminate the steam. This fraction is determined by the ultimate composition of the lignocellulosic biomass and carbonaceous feedstock. A typical minimum fraction to get complete carbon conversion is 10-30% of biomass by weight in the 25 carbonaceous feedstock/biomass mixture; however, any fraction between 10% to 50% may be used. The above object is achieved with the following process. A process for controlling the carbon conversion of a 30 gasifier fuelled with a carbonaceous feedstock by mixing in biomass, the process comprising the steps of (a) pressurizing the biomass and carbonaceous feedstock; WO 2012/084953 - 3 - PCT/EP2011/073417 (b) introducing the biomass and carbonaceous feedstock into the gasification reactor vessel; (c) partially oxidizing the carbonaceous feedstock/biomass with a molecular oxygen-comprising gas 5 to obtain a synthesis gas comprising carbon monoxide and hydrogen; (d) measuring the C02 content of the syngas and comparing with a pre-determined value range; (e) adjusting the biomass/carbonaceous feedstock ratio 10 by changing the biomass feed rate; wherein said gasifier feed comprises from a minimum of 10 wt% to 50 wt%, preferably 10 wt% to 30 wt% of biomass and wherein the level of biomass may adjusted within this range as needed to control the carbon conversion and 15 reduce or eliminate the moderator gas demand. The synthesis gas produced by this process may also contain water, hydrogen sulphide and carbon dioxide. Applicant found that by mixing 10 wt% to 50 wt%, preferably 10 wt% to 30 wt% biomass with the carbonaceous 20 feedstock, the temperature in the gasifier can be controlled using the oxygen/carbonaceous fuel ratio and the carbon conversion can be controlled using the biomass/carbonaceous fuel ratio, thereby simplifying process control. The incorporation of biomass as 25 described has the additional beneficial effects of saving on steam and boiler feed water consumption and also reduces the overall carbon footprint of the gasifier. The invention will be described in more detail below. 30 In step (a) pressurization may be accomplished by a lock hopper system or (solids) pumps. A lock hopper system is generally used to pressurize dry and solid feedstocks. A pump can be used to pressurize liquid or WO 2012/084953 - 4 - PCT/EP2011/073417 slurry (mixture of solids and liquid) feedstocks. A solids pump can also be applied to pressurize solid feedstocks. In step (b) pulverized or liquid/slurry biomass can 5 be mixed with the carbonaceous feed or fed separately to the gasifier. A separate feedline for the biomass to the gasification reactor is preferred as this will reduce the response time on a step change from the carbon conversion controller significantly. Mixing of the biomass with the 10 carbonaceous feed can be done at several locations in the fuel preparation and pressurization section. In step (c) any type of coal or oil burner configurations can be used. When the biomass is supplied separately to the gasification reactor then the biomass 15 may also be introduced through a nozzle in the burner area of the gasification reactor. The oxygen for the partial oxidation of the biomass will then need to be supplied via the carbonaceous fuel burner. In step (c) the carbonaceous/biomass feedstock is 20 subjected to partial oxidation with a molecular oxygen comprising gas. The partial oxidation is preferably performed at a temperature of between 1000 and 1800 'C and more preferably at a temperature between 1200 and 1800 'C. The pressure at which partial oxidation is 25 performed is preferably between 0.3 and 12 MPa and preferably between 3 and 10 MPa. When an ash containing feedstock is used the temperature conditions are so chosen that a slag layer will form on the interior of the reactor vessel in which the partial oxidation takes 30 place. In step (d) the C02 content is preferably measured in cleaned and cold syngas, for example downstream of the wet scrubber. A fast measurement is preferred to minimize WO 2012/084953 - 5 - PCT/EP2011/073417 the time between a step change in C02 concentration and the control action. An infrared based analyser is an example of such a fast measurement device. In step (d) the C02 content of the syngas is 5 compared with a pre-defined value. In step (e) a controller will subsequently adjust the biomass/coal ratio by preferably changing the set-point of the biomass feed rate controller. The carbonaceous feedstock is preferably coal, as for 10 example anthracite, brown coal, bitumous coal, and sub bitumous coal. Examples of alternative carbonaceous feedstocks are petroleum coke, peat and heavy residues as extracted from tar sands or the asphalt fraction as separated from said residues in a de-asphalting process. 15 Residues from refineries such as residual oil fractions boiling above 360 'C, directly derived from crude oil, or from oil conversion processes such as thermal cracking, catalytic cracking, hydrocracking etc may also be used as the carbonaceous feedstock. 20 Any biomass derived feedstocks containing low moisture content of typically lower than 20 wt% and which can be ground to particles with sizes between 10 and 1000 micron are suitable solid biomass feedstocks comprising the 10 wt% to 50 wt% biomass component of the 25 carbonaceous feedstock. Feedstocks obtained by torrefaction of a biomass source are suitable as well and are preferred at higher biomass fractions in the mixture. Torrefaction is preferably combined with a compression or pelletization step in order to make the biomass feed more 30 suited for a gasification process wherein the biomass feed is supplied in a so-called dry form or slurry form when torrefied particles are mixed with a carbonaceous liquid. Torrefaction of biomass source material is well WO 2012/084953 - 6 - PCT/EP2011/073417 known and for example described in M. Pach, R. Zanzi and E. Bjbrnbom, Torrefied Biomass a Substitute for Wood and Charcoal. 6th Asia-Pacific International Symposium on Combustion and Energy Utilization. May 2002, Kuala Lumpur 5 and in Bergman, P.C.A., "Torrefaction in combination with pelletisation - the TOP process", ECN Report, ECN-C-05-073, Petten, 2005. Another suitable solid biomass fuel in the present process is obtained by drying and slow pyrolysis of a 10 biomass source. In slow pyrolysis processes a solid char feed component is typically obtained. Slow pyrolysis is well known and for example described In: "Pyrolysis and Other Thermal Processing". US DOE 14-08-2007. A suitable liquid or solid biomass feedstock for use 15 in the present process is obtained by drying and flash pyrolysis of a biomass source. In flash pyrolysis processes a solid char and a liquid biomass feed component are typically obtained. Both can be used as feedstock for the gasification process. Flash pyrolysis 20 is well known and for example described in EP-A-904335; in Dinesh Mohan, Charles U. Pittman, Jr., and Philip H. Steele. Pyrolysis of Wood/Biomass for Bio-oil: A Critical Review. Energy & Fuels 2006, 20, 848-889; and in E. Henrich: Clean syngas from biomass by pressurised 25 entrained flow gasification of slurries from fast pyrolysis. In: Synbios, the syngas route to automotive biofuels, conference held from 18-20 May 2005, Stockholm, Sweden (2005). The present invention is also directed to embodiments wherein a so-called biomass slurry is used as 30 feedstock. The slurry can be obtained by mixing the pyrolysis oil and char. Suitable biomass sources are weeds or residues of the agricultural industry. Examples of suitable residue WO 2012/084953 - 7 - PCT/EP2011/073417 products are streams generated in the palm oil industry, corn industry, bio-diesel industry, forestry industry, wood processing industry and paper industry. Certain biomass is relatively expensive-for example, wood 5 pellets. So, in order to keep operational costs as low as possible, less costly biomass sources are preferred, such as oil palm trunks and saw dust. However, the low quantities of biomass needed for the replacement of the moderator steam increases the chance for the application 10 of biomass from waste streams, which are normally cheaply available in small quantities. The present invention now provides for a process whereby the partial oxidation of a carbonaceous feedstock is performed in an efficient manner thereby obtaining 15 synthesis gas with a reduced carbon footprint suited for catalytic conversion reactions. An especially interesting catalytic conversion reaction is a hydrocarbon synthesis process. In a hydrocarbon synthesis process, synthesis gas is catalytically converted into hydrocarbon compounds 20 ranging from methane to high molecular weight molecules comprising up to 200 carbon atoms, or, under particular circumstances, even more. An example of a hydrocarbon synthesis process is the Fischer-Tropsch process, described in e.g. WO 02/02489, WO 01/76736, WO 02/07882, 25 EP 510771 and EP 450861. Step (c) may be performed by means of various gasification processes, such as for example the so-called moving bed process, fluid bed gasifier process or the entrained-flow gasifier process as for example described 30 in Gasification, by Christofer Higman and Maarten van der Burgt, 2003, Elsevier Science, Burlington MA, Pages 85-128. Preferably an entrained-flow gasifier is used because the process can handle a large variety of WO 2012/084953 - 8 - PCT/EP2011/073417 feedstock and because a tar-free synthesis gas is prepared. In such a process the feedstock and oxygen are introduced into the reactor co-currently, preferably by means of a suitable burner. Examples of suitable burners 5 and their preferred uses are described in US-A-4510874 and in US-A-4523529. The operating conditions are such that the process is operated in a slagging mode, which means that the operating temperature is above the ash melting point. Suitably the carbonaceous feedstock and 10 the molecular oxygen comprising gas is converted to synthesis gas by providing said reactants to a burner as present in a gasification reactor at a pressure of between 3 and 10 MPa and preferably at a pressure between 4 and 8 MPa. The operating temperature is suitably 15 between 1200 and 1800 0 C. The synthesis gas is preferably cooled to a temperature of below 1000 0 C, preferably below 500 0 C with either direct quenching with evaporating water, direct quenching with a methanol-water mixture, by indirect heat exchange against evaporating water or 20 combination of such cooling steps. Slag and other molten solids are suitably discharged from the gasification reactor at the lower end of the said reactor. A solid carbonaceous feedstock/biomass feed mixture may be provided to the burner of the entrained flow 25 gasifier reactor as a slurry in water. Coal slurry feeding processes are for example described in EP-A 168128. Preferably the solid carbonaceous feedstock/biomass feedstock is provided to the burner in a gas-solids mixture comprising the solid feed in the 30 form of a powder and a suitable carrier gas. Suitable carrier gasses are nitrogen, carbon dioxide, natural gas or synthesis gas, i.e. a mixture comprising of CO and H 2 The carrier gas is preferably carbon dioxide. The use of WO 2012/084953 - 9 - PCT/EP2011/073417 this carrier gas is for example described in WO-A 2007042562. Figure 1 shows a preferred embodiment of the present invention. 5 Figure 1 schematically shows a system for producing and cleaning synthesis gas. In a gasification reactor 9 a carbonaceous feed, a biomass feed and an oxygen containing feed are introduced. The oxygen and carbonaceous stream are fed via lines 19 and 8 10 respectively. The biomass can be fed separately via line 25 to the gasification reactor 9 but can also be mixed with the carbonaceous stream in the feeding and pressurization section 7 via line 5. The carbonaceous stream 8 and biomass stream 25 are at least partially 15 oxidised in the gasification reactor 9, thereby obtaining a raw synthesis gas 10 and a slag. To this end usually several burners (not shown) are present in the gasification reactor 9. The biomass stream 25 can also be introduced in the gasification reactor 9 via nozzles in 20 the burner area of gasification reactor, for example at burner level. Then the oxygen for the partial oxidation of the biomass will be added to the burner oxygen of the carbonaceous stream 8. The carbon conversion of the gasifier is controlled 25 with controller 18. Input for the carbon conversion controller is the measured C02 content of the syngas 16 downstream of the wet scrubber 15 with the C02 analyser 17. As soon as the C02 content drops below a pre determined value than the biomass/coal ratio needs to be 30 increased by changing the biomass feed 3 and increasing the flow rate of line 5 or 25. As soon as the C02 concentration exceeds a maximum pre-determined value, then the biomass/coal ratio can be reduced by changing WO 2012/084953 - 10 - PCT/EP2011/073417 the biomass feed 3 and decreasing the flow rate of line 4 or 25. The C02 analyser and benefits for controlling the steam and oxygen to carbon (0/C) ratio are described in W02008125556A1 and W0200768684A2. 5 An advantage for the use of biomass to control the carbon conversion is that a C02 neutral feedstock is used, which reduces the C02 footprint of the plant. In addition this application reduces the high pressure steam and high quality boiler feed water requirements for the 10 gasification island. The produced raw synthesis gas is fed via line 10 to a cooling section 11; herein the raw synthesis gas is usually cooled to about 200-400 0 C. The cooling section 11 may be an indirect heat exchanger or a quench vessel. 15 In case of a quench vessel liquid water is preferably injected via line 23 into a synthesis gas stream. Liquid water is preferably injected in the form of a mist. The cooling section 11 can be integrated in the gasification reactor pressure vessel at the bottom of the 20 reactor or installed as a separate vessel. When the syngas and slag both exit at the bottom of the gasification reactor then the cooling section 11 is integrated in the pressure vessel and both the syngas and fly ash and slag will enter the cooling section. When the 25 syngas leaves the gasification reactor 9 at the top and slag at the bottom then only syngas with fly ash will enter the cooling section that is installed as separate section downstream of the reactor. As shown in the embodiment of Figure 1, the raw 30 synthesis gas leaving the cooling section 10 is further processed. To this end, it is fed via line 12 into a dry or wet solids removal unit 13 to at least partially remove ash in the raw synthesis gas. As the solids WO 2012/084953 - 11 - PCT/EP2011/073417 removal unit 13 is known to those of ordinary skill in the art, it is not further discussed here. The ash is removed from the solids removal unit via line 24. After the solids removal unit 13 the raw synthesis gas may be 5 fed via line 14 to a wet gas scrubber 15 and subsequently via line 16 to battery limits. After further syngas treating, the residual syngas can be used for several applications like power generation, production of H2, fertilizer, Fischer Tropsch liquids and other chemicals. 10 The reduction of the moderator steam with the addition of biomass to coal was modelled using a computer simulation. For the gasification of a selected Drayton coal, approximately 76 kg steam per tonne coal will be required to obtain full carbon conversion at a 15 gasification temperature of 1500 0 C. The addition of coal with wood will gradually reduce the moderator requirement as shown in table 1. With the replacement of >25% of the coal with wood, moderator steam can be eliminated. 20 Table 1--Typical moderator consumption at different biomass fractions in biomass/coal mixture Coal in mixture 100% 90% 80% 70% Biomass in mixture 0% 10% 20% 30% Gasification conditions Temperature oC 1500 1500 1500 1500 Pressure bara 41 41 41 41 GASIFIER INPUT Carrier kg/s 2.7 2.7 2.7 2.7 Moderator stearn kg/s 21 1.1 0.2 O 0 02 kg/s 22.2 21.3 20.4 19.7 Fuel kg/s 27.6 27.6 27.6 27.6 GASIFIER OUTPUT Syngas kg/s 49.9 48.5 47.1 46.6 Slag kg/s 6.0 5.5 4.9 4.4

Claims (14)

1. A process for controlling the carbon conversion of a gasifier fuelled with a carbonaceous feedstock by mixing in biomass, the process comprising the steps of (a) pressurizing the biomass and carbonaceous 5 feedstock; (b) introducing the biomass and carbonaceous feedstock into the gasification reactor vessel; (c) partially oxidizing the carbonaceous feedstock/biomass with a molecular oxygen-comprising gas 10 to obtain a synthesis gas comprising carbon monoxide and hydrogen; (d) measuring the C02 content of the syngas and comparing with a pre-determined value range; (e) adjusting the biomass/carbonaceous feedstock 15 ratio by changing the biomass feed rate; wherein said biomass and carbonaceous feedstock comprises from 10 wt% to 50 wt% of biomass and wherein the level of biomass is adjusted within this range to control the carbon conversion. 20

2. Process according to claim 1, wherein the carbonaceous feedstock and biomass are separately introduced into the gasification reactor.

3. Process according to claim 1, wherein the carbonaceous feedstock and biomass are introduced in the 25 gasification reactor as a mixture.

4. Process according to any one of claims 1-3, wherein the feedstock to the gasifier comprises from 10 wt% to 30 wt% of biomass fuel.

5. Process according to any one of claims 1-4, wherein 30 the biomass is a solid biomass as obtained by torrefaction. WO 2012/084953 - 13 - PCT/EP2011/073417

6. Process according to any of claims 1-5, wherein the biomass is a solid biomass as obtained by slow pyrolysis of a biomass source.

7. Process according to any one of claims 1-6, wherein 5 the biomass is a solid biomass as obtained by flash pyrolysis of a biomass source.

8. Process according to claim 4, wherein the biomass is a liquid biomass as obtained by flash pyrolysis of a biomass source. 10

9. Process according to claim 4, wherein the biomass is a slurry biomass consisting of a mixture of pyrolysis oil and char as obtained by flash pyrolysis of a biomass source.

10. Process according to any one of claims 1-9 wherein 15 the carbonaceous feedstock is coal.

11. Process according to any one of claims 1-9 wherein the carbonaceous feedstock is oil residue.

12. Process according to any one of claims 1-11, wherein in step (c) the carbonaceous feedstock/biomass fuel 20 mixture and the molecular oxygen comprising gas are converted to synthesis gas by providing said reactants to a burner as present in a gasification reactor at a pressure of between 1 and 10 MPa.

13. Process according to any one of claims 1-12, wherein 25 in step (d) the C02 content is measured using an infrared based analyser.

14. Process according to any one of claims 1-13, wherein, in step (e) a controller will adjust the biomass/coal ratio by changing the set-point of a biomass 30 feed rate controller.